The present embodiments relate to a method for MR image data acquisition.
Magnetic resonance (MR) technology is a known technology that enables images of the interior of an examination subject to be produced. The examination subject is positioned in a magnetic resonance unit in a comparatively strong static, homogeneous basic magnetic field (e.g., a B0 field) having field strengths of 0.2 tesla to 7 tesla and more. This provides that the nuclear spins thereof are oriented along the basic magnetic field. In order to trigger nuclear spin resonances (e.g., nuclear spin signals), high-frequency excitation pulses (e.g., HF excitation pulses) or high-frequency pulses are irradiated into the examination subject, the triggered nuclear spin resonances are measured, and on the basis thereof, MR images are reconstructed or spectroscopy data is ascertained. For the purpose of position encoding of the measurement data, rapidly switched magnetic gradient fields (e.g., gradients) are superimposed on the basic magnetic field. The measurement data recorded is digitized and stored as complex numerical values in a k-space matrix. From the values contained in the k-space matrix, an associated MR image may be reconstructed, for example, by a multidimensional Fourier transform.
With regard to the triggering of the nuclear spin signals, the spins situated in the examination region are excited from the state of equilibrium by the HF excitation pulses and tilted into the transverse plane. This transverse magnetization may be measured by induction.
With regard to the excitation, selective HF excitation pulses that, for example, excite only one layer in the examination subject may be distinguished from non-selective HF excitation pulses. Non-selective HF excitation pulses may excite the entire examination subject, or at least the examination region to be examined in the examination subject, in a uniform fashion. Additional gradients are switched for the purpose of spatial resolution. For a resolution in the layer direction, for example, gradients are switched in the layer direction.
MR sequences that may use non-selective HF excitation pulses are already known. For example, a rapid signal point (RASP) sequence is an example, as is described by Heid and Deimling in “Rapid Signal Point (RASP) Imaging,” SMR, 3rd Annual Meeting, p. 684, 1995. Other examples include, for example, turbo spin echo sequences or also ultrashort echo time (UTE) sequences, as is described, for example, in the paper by Sonia Nielles-Vallespin, “3D radial projection technique with ultrashort echo times for sodium MRI: Clinical applications in human brain and skeletal muscle,” Magn. Res. Med. 2007, 57, pp. 74-81.
Magnetic resonance examinations may be very loud. The main reason for this is rapidly changing gradient fields that result in distortions and oscillations in the gradient coil and the transmission of this energy to the housing. In order to design a sequence that is used as quietly as possible, the changes in the gradients over time dG/dt should be as small as possible.
Certain MR sequences use imaging gradients that are already switched on at the time of the excitation. These sequences may have an ultrashort echo time. Examples are the aforementioned RASP and UTE sequences as well as single-point ramped imaging with Ti enhancement (SPRITE), sweep imaging with Fourier transofmr (SWIFT), or pointwise encoding time reduction with radial acquisition (PETRA) sequences. The PETRA sequence is known, for example, from Grodzki, D M, Jakob, P M, and Heismann, B, in “Ultrashort echo time imaging using pointwise encoding time reduction with radial acquisition (PETRA),” Magn. Reson. Med. 67(2), 510-8, Jun. 30, 2011.
The stated sequences may measure in the steady state, which the spin system reaches during the measurement given a constant flip angle and repetition time (TR). With regard to certain measurements such as, for example, anisotropic resolution or combination of different measurement trajectories, the time used for the data acquisition does, however, also change the relative gradient distances between the repetitions during the measurement.
One advantage of sequences with imaging gradients switched on for excitation purposes is that the sequences may be very quiet. The reason for this is the fact that the gradient distances between the repetitions are for the most part small, and with a sufficiently large ramp or rise duration, noises from the gradient system may be ignored or not noticed.
In order to obtain as small a noise development as possible from the gradient system, the ramp times or rise times are chosen of such a length that the ramp times and the rise times are also sufficiently large in the sequence regions having greater gradient distances between the repetitions and a maximum data acquisition duration. The minimum possible repetition time thus increases.